Capitalist production requires exchange relations, commodities, and money, but its differentia specifica is the purchase and sale of labor power.  For this purpose, three basic conditions become generalized throughout the society.  First, workers are separated from the means with which production is carried on, and can gain access to them only by selling their labor power to others.  Second, workers are freed of legal constraints, such as serfdom or slavery, that prevent them from disposing of their own labor power.  Third, the purpose of the employment of the worker becomes the expansion of a unit of capital belonging to the employer, who is thus functioning as a capitalist.  The labor process therefore begins with a contract or agreement governing the conditions of the sale of labor power by the worker and its purchase by the employer (35-36).